Our first-principles calculation shows that molecular hydrogen (H(2)) adsorption at an exposed Fe(II) site in metal-organic frameworks could induce a spin flip in the Fe(II) center resulting in a spin-state transition from a triplet high-spin (HS) to a singlet low-spin (LS) state. The Kubas-type Fe-H(2) interaction, where H(2) coordinates onto the Fe(II) center as a σ-ligand, is found commensurate in strength with the exchange interaction of Fe 3d electrons, which is responsible for the occurrence of the spin-state transition in this system. The H(2) binding energies are 0.08 and 0.35 eV per H(2) at the HS and LS states, respectively. This effect is expected to find applications in spin-control in molecular magnets, hydrogen sensing and storage.